Gaseous fuel engines are known for their ability to burn clean relative to liquid fuel compression ignition engine counterparts. Gaseous fuels, however, are also well known for difficulty in attaining successful ignition. Some gaseous fuel engines utilize a spark plug, whereas other gaseous fuel engines utilize a small amount of a pilot fuel such as distillate diesel fuel, compression ignited to initiate combustion of a larger main charge of gaseous fuel such as natural gas. In these latter engines, the gaseous fuel may be supplied to the engine intake manifold, or metered directly into individual cylinders, where is it mixed with air prior to being ignited responsive to the pilot diesel injection.
In many dual fuel engine systems, injection of the liquid fuel and gaseous fuel is controlled by two separate needle checks within a fuel injector connected to both gaseous fuel and liquid fuel common rails. Designs are known where concentric needle checks are used, with one of an inner check and an outer check used to open and close a gaseous fuel outlet, and the other of the inner and outer check used to open and close a liquid fuel outlet. In other systems, adjacent rather than coaxial needle checks are used. One known adjacent needle check design employs hydraulic control pressure from a liquid fuel common rail applied to a top end of each of the needle checks opposite the working tip which controls opening and closing of the corresponding nozzle outlet. Liquid rail pressure is also applied to an opening hydraulic surface of the check controlling liquid fuel injection. In the case of the check controlling gaseous fuel injection, a pressure of the gaseous fuel common rail is applied to a primary opening hydraulic surface of the check. In still another known system, U.S. Pat. No. 7,627,416 appears to teach a dual fuel common rail design in which liquid diesel fuel and natural gas fuel are both injected from a single fuel injector associated with each engine cylinder. This reference recognizes there may be instances in which the engine will need to operate solely on liquid diesel fuel due to exhaustion of the natural gas fuel supply or possibly some fault in the natural gas portion of the system. Certain problems and challenges associated with liquid fuel only operation are neither recognized nor addressed, however.